Polycrystalline Zn Research Articles

Fe dopant in ZnO: 2+ versus 3+ valency and ion-carrier s,p−d exchange interaction

Dopants of transition metal ions in II-VI semiconductors exhibit native 2+ valency. Despite this, 3+ or mixed 3+/2+ valency of iron ions in ZnO was reported previously. Several contradictory mechanisms have been put forward for explanation of this fact so far. Here, we analyze Fe valency in ZnO by complementary theoretical and experimental studies. Our calculations within the generalized gradient approximation (GGA+U) indicate that the Fe ion is a relatively shallow donor. Its stable charge state is Fe2+ in ideal ZnO, however, the high energy of the (+/0) transition level enhances the compensation of Fe2+ to Fe3+ by non-intentional acceptors in real samples. Using several experimental methods like electron paramagnetic resonance, magnetometry, conductivity, excitonic magnetic circular dichroism and magneto-photoluminescence we confirm the 3+ valency of the iron ions in polycrystalline (Zn,Fe)O films with the Fe content attaining 0.2%.We find a predicted increase of n-type conductivity upon the Fe doping with the Fe donor ionization energy of 0.25 +/- 0.02 eV consistent with the results of theoretical considerations. Moreover, our magnetooptical measurements confirm the calculated non-vanishing s,p-d exchange interaction between band carriers and localized magnetic moments of the Fe3+ ions in the ZnO, being so far an unsettled issue.

Structural and optical analyses of polycrystalline Zn1−xSbxSe thin films prepared by resistive heating technique

Here we report the influence of Sb doping on the structural and optical properties of Zn1−xSbxSe (0⩽x⩾0.15) thin films prepared by thermal evaporation technique on glass substrate. Various characterization techniques such as X-ray diffraction (XRD), EDS, Raman spectroscopy and spectroscopic ellipsometer are employed to assess the structural and optical properties of the deposited films. XRD analysis reveals the formation of polycrystalline cubic structure having preferred growth orientation along (111) plane without any evidence of secondary phases. Crystallographic parameters like grain size, micro strain, dislocation density, number of crystallites per unit area and texture coefficient point out the structural modification in ZnSe films with Sb inclusion. Raman analysis shows the existence of three 1LO, 2LO and 3LO phonon modes at 251, 511 and 745cm−1 in pure ZnSe while 3LO mode disappears by the incorporation of Sb atoms in ZnSe matrix. Increase in FWHM of Raman peaks with Sb concentration also indicates the change in crystalline quality of ZnSe films which is in accordance with our XRD results. Spectroscopic ellipsometry results demonstrate a decreasing trend for the optical band gap energy (from 2.61eV to 1.81eV) with increasing Sb content.

Improved Performances of Ethanol Sensor Fabricated on Al-Doped ZnO Nanosheet Thin Films

The effect of Al-doped ZnO (AZO) to the sensitivity improvement of ethanol vapor sensor was investigated. The pure ZnO and AZO thin films were synthesized by chemical bath deposition method. Analysis of the crystal structure, chemical composition, and morphology of the samples are carried out using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and scanning electron microscopy (SEM). XRD results show that the phase formed is polycrystalline Zn, while the EDS measurements show that the composition of after synthesized AZO were 1.48 at%, 2.90 at%, and 3.55 at%. The SEM results showed flowerlike microstrcuture and nanosheet shaped are formed in the resulting thin films, which are captured by the formation of sheets of transparent structure in AZO. Moreover, the resulting ZnO and AZO sensors were exposed to the different concentration of ethanol vapor as much as 200, 400, and 600 ppm, respectively. The measurement of vapor sensor system performance was tested to analyze the sensitivity and the responsiveness of the AZO thin films. The sensors have highest sensitivity at certain amount of Al doping. It is obtained that the optimum sensitivity of the sensor is investigated at the sample of 2.9 at% Al-ZnO. The sensitivity for pure ZnO sample at concentration of 200, 400, and 600 ppm of ethanol were 70.88%, 78.21%, and 88.57%, respectively. The highest sensitivity for AZO samples is 95.29% at low ethanol concentrations (200 ppm) and 96.68% at the high concentration (parts per million), respectively.

Room-temperature ferromagnetism in polycrystalline Zn1–xFexO (0≤x≤0.075) solid solutions synthesized by the precursor method

Polycrystalline Zn1–xFexO (0 ≤ x ≤ 0.075) solid solutions with the wurtzite structure belonging to the class of diluted magnetic semiconductors were synthesized by the precursor method using mixed Zn1–xFex(HCOO)2·2H2O formate as a precursor. As an example we show for samples with x = 0.025 and 0.05 that the room-temperature magnetic moment of Zn1–xFexO solid solutions increases regularly with the iron concentration.

Observation of ferromagnetism at room temperature in polycrystalline Zn1 − x Fe x O solid solutions synthesized by the precursor method

Polycrystalline Zn1 − x Fe x O (0 ≤ x ≤ 0.075) solid solutions with the wurtzite structure, which belong to the class of diluted magnetic semiconductors, have been synthesized by the precursor method using a mixed formate of the composition Zn1 − x Fe x (HCOO)2 · 2H2O as a precursor. It has been found that the iron concentration in Zn1 − x Fe x O solid solutions, which exhibit ferromagnetic properties with a high degree of magnetization at room temperature, is determined by two factors: (1) the degree of substitution of iron for zinc in the structure of zinc formate Zn(HCOO)2 · 2H2O and, therefore, in the structure of zinc oxide ZnO and (2) the conditions of heat treatment of the precursor. It has been shown using samples with x = 0.025 and 0.05 as an example that the magnetization of Zn1 − x Fe x O solid solutions regularly increases with an increase in the iron concentration. Based on the results of the X-ray photoelectron spectroscopy analysis of Zn1 − x Fe x O powders, it has been suggested that the iron concentration increases in the surface layer of particles, and the oxidation state of iron is 3+.

Structure, morphologies and dye removal efficiency of ZnO nanorods grown on polycrystalline Zn substrate

Rod-like ZnO with the different morphologies were grown on polycrystalline Zn substrate by a simple hydrothermal process in a NaOH or NH4OH solution at the hydrothermal temperature range from 80 to 150°C for different reaction time. Variations preparation in the different alkali solution concentration, hydrothermal temperature, and reaction times were explored to shed light on the morphology of the rod-like nanostructures. The thorough structural characterization including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction, and X-ray photoelectron spectrum (XPS) were employed to examine the morphology and the microstructure of the final products. It was found that alkali solution concentration, hydrothermal temperature and time have important influence on the morphology of the rod-like nanostructures. The dye removal efficiency of ZnO nanorods was explored by the decoloration of azo dye Congo red (CR). In order to obtain the optimum removal conditions of Congo red, the performance of removing CR with various initial concentrations by ZnO nanorods on Zn substrates with different morphologies was tested under various ambient conditions (visible light illumination and darkness). All prepared samples showed an excellent dye removal efficiency for organic pollutants CR from wastewater, making them promising candidates for the wastewater treatment.

Investigation of Porous Zn Growth Mechanism during Zn Reactive Sputter Deposition

(Received March 22, 2013) Ar O Zn plasma discharges created during DC reactive magnetron sputtering of a Zn target and RF reactive magnetron sputtering of a ceramic ZnO target were investigated and compared by means of the Langmuir probe measurements in order to determine the mechanism of growth of porous Zn lms during DC-mode Zn reactive sputtering. The power supplied to the magnetrons during the sputtering was kept at 125 W and the plasma was characterised as a function of oxygen content in the sputtering gas mixture, ranging from 0 to 60% for two gas pressures related to porous Zn lm deposition, namely 3 mTorr and 5 mTorr. Based on the correlation of plasma properties measurements with scanning electron microscope imaging and X-ray di raction of the lms deposited under selected conditions it was found that the growth of porous, polycrystalline Zn lms was governed by high electron density in the plasma combined with a high electron temperature and an increased energy of the ions impinging on the substrate.

Carbon ion beam-induced variation in orientation of crystal planes of polycrystalline Zn nanowires

In the present work, polycrystalline Zn nanowires are synthesised within polymeric templates by the electrochemical deposition technique. Free-standing Zn nanowires with diameters 100, 200, 400 nm were synthesised on copper substrate acting as cathode, using this technique. The synthesised nanowires were irradiated with 40 MeV C4+ ion beam for various fluencies at Inter University Accelerator Centre, New Delhi, India. The effect of carbon ion beam on the grain size and preferred orientation of crystal planes of Zn nanowires were studied. The Rigaku X-ray diffractometer was used for X-ray diffraction (XRD) spectra of pristine and irradiated Zn nanowires. Post irradiation XRD analysis showed variation in the intensity of peaks, indicating the change in the orientation of crystal planes of the material. The preferred orientation of the crystal lattice planes in the polycrystalline material was found using texture analysis. To quantitatively investigate the degree of preferred orientations, the texture coefficients (TC) were calculated. The variation of TC was used to demonstrate the effect of irradiation on preferred orientation.

Synthesis and characterization of highly preferred orientation polycrystalline Co-doped ZnO thin films prepared by improved sol–gel method

Highly preferred orientation polycrystalline Zn1−xCoxO (x = 0, 0.03, 0.06, 0.09) thin films were prepared by improved sol–gel method on quartz glass substrates. The structural, optical and magnetic properties were investigated. The X-ray diffraction patterns show that all the samples have the same structure with one highly oriented c-axis (002) peak. None of the samples showed any signal of impurity phases. The c-axis lattice constant increased linearly with the increase in Co doping content, indicating that the doping of Co ions into the host lattice did not change the wurtzite structure of ZnO. UV–Vis transmittance spectroscopy showed that the average optical transmittance of the films is about 90 % in visible wavelength range. The optical band gap (Eg) decreased with increasing Co content. Also, the results of vibration sample magnetization ascertained the ferromagnetic behavior of Co-doped ZnO, having a Curie temperature higher than room temperature.

Modifications induced by silicon and nickel ion beams in the electrical conductivity of zinc nanowires

This paper presents the modification in electrical conductivity of Zn nanowires under swift heavy ions irradiation at different fluences. The polycrystalline Zn nanowires were synthesized within polymeric templates, using electrochemical deposition technique and were irradiated with 80 MeV Si7+ and 110 MeV Ni8+ ion beams with fluence varying from 1 × 1012 to 3 × 1013 ions/cm2. I–V characteristics of exposed nanowires revealed a decrease in electrical conductivity with increase in ion fluence which was found to be independent of applied potential difference. But in the case of high fluence of Ni ion beam (3 × 1013 ions/cm2), electrical conductivity was found to increase with potential difference. The analysis found a significant contribution from grain boundaries scattering of conduction electrons and defects produced by ion beam during irradiation on flow of charge carriers in nanowires.

Photoluminescence Properties of Cd Doped ZnO Films Obtained by PLD

Ternary polycrystalline Zn1xCdxO semiconductor films with cadmium content (x) ranging from 0 to 0.23 were obtained on quartz substrate by pulse laser deposited (PLD) technique. X-ray diffraction measurement revealed that all the films were single phase of wurtzite structure grown on c-axis orientation with its c-axis lattice constant increasing as the Cd content x increasing. Atomic force microscopy observation revealed that the grain size of Zn1xCdxO films decreases continuously as the Cd content x increases. Photoluminescence measurements showed that the band gap decreases from 3.27 to 2.78 eV with increasing the Cd content x. Increasing the Cd content x also leads to the broadening of the emission peak. The shift of PL emission to visible light as well as the decrease of resistivity makes the Zn1xCdxO films potential candidate for optoelectronic device.

Raman scattering spectra and magnetic properties of polycrystalline Zn1−xCoxO ceramics

Six polycrystalline ceramic samples of Zn1−xCoxO (x=0, 0.02, 0.05, 0.1, 0.2, and 0.3) prepared by conventional solid-state reaction have been studied the structural characterization, and optical and magnetic properties at room-temperature. Analyses of x-ray diffraction (XRD) patterns indicated the substitution of Co2+ for Zn2+ in the ZnO-wurtzite structure, and the presence of a secondary phase of ZnCo2O4 spinel. These factors directly influenced the features of Raman spectra of the samples in both the cases of non-resonant and resonant scattering, corresponding to excitation wavelengths of 488 and 325nm, respectively. Besides conventional Raman scattering (RS) modes associated with the ZnO structure, there are additional modes associated with the incorporation of Co dopants into the ZnO host lattice, and with ZnCo2O4 spinel. Particularly, under the resonant condition we have observed the longitudinal-optical (LO) phonon and its overtones up to the seventh order in the samples doped with high Co concentrations. This indicates the existence of a large-deformation potential caused by the displacement of atoms from their equilibrium positions due to Co dopants, which affects the electronic band structure of Zn1−xCoxO. Magnetic measurements revealed the ferromagnetic order in Zn1−xCoxO as x≥0.2. Basing on the results obtained from the XRD and RS analyses, and from magnetic studies of reference samples ZnCo2O4 and Co3O4, we believe that the ferromagnetism in Zn1−xCoxO with x≥0.2 is related to exchange interactions between Co2+ ions mediated by extrinsic defects.

From porous to dense thin ZnO films through reactive DC sputter deposition onto Si (100) substrates

AbstractThin polycrystalline ZnO films are deposited onto Si (100) substrates by means of DC reactive sputter deposition from a Zn target in an argon–oxygen mixture. The influences of the oxygen content in the mixture and of the total gas pressure in the reactor during deposition as well as of applying postgrowth annealing in an oxygen flow at elevated temperatures on the structural, chemical, and electrical properties of the obtained films are discussed. Three types of thin‐film structures were obtained: a porous polycrystalline Zn layer, a porous polycrystalline ZnO layer and a dense ZnO film. The porous Zn layer grew at oxygen content less than 33% and upon annealing in an oxygen flow at 400, 600, and 800 °C transformed to porous polycrystalline ZnO films. For oxygen contents ≥33% the obtained films were dense and polycrystalline with a 00.2 dominating orientation. X‐ray diffraction (XRD), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), and Hall effect measurements were applied to characterize the obtained films.

Pareto-optimal analysis of Zn-coated Fe in the presence of dislocations using genetic algorithms

To design a coating that will absorb maximum energy prior to failure with minimum deformation, the shearing process of polycrystalline Zn coated Fe is simulated in the presence of dislocations, using molecular dynamics. The results fed to an Evolutionary Neural Network generated the meta-models of objective functions required in the subsequent Pareto-optimization task using a Multi-objective Genetic Algorithm. Similar calculations conducted for single crystals, and also in the absence of dislocations, are compared and analyzed.

Magnetic Property Study of Nickel Cerium Doped Zinc Ferrite Nano Particles

Nano-particles of polycrystalline Zn Fe2 O4 doped with, Ni and Ce (Zn Niy Cex Fe2-x-y O4, where x =0.01, 0.012, 0.014, 0.016, y=0.003) was prepared by sol-gel auto combustion route. The microwave sintered ferrite was characterized and the nano size was confirmed by XRD and the SEM monographs. The EDAX studies confirm the composition of Ni, Ce doped Zn-ferrites and VSM studies show the behaviour of coercivity and Saturation magnetization. The permittivity increased with the increase in Ni-Ce doping concentration.

"Magnetic Properties of Nano Crystalline Nickel, Samariumdoped Zinc Ferrite"

Nano-particles of polycrystalline Zn Fe2 O4 are prepared using sol-gel method with Ni, and Sm. They are obtained as dried gel after the successful chemical reaction of their respective metal nitrate solutions in the midst of citric acid as catalyst. Synthesis of materials is confirmed using XRD from the report of single phase polycrystalline ferrite material. The magnetic properties of Zn-ferrite ceramics Zn1-x-y NiySmx Fe2 O4, (where x = 0.01, 0.012, 0.014, 0.016, y=0.001) were synthesized by sol gel auto combustion method. The structure and composition of Sm doped Zn-ferrite ceramics were analyzed and the nano size was confirmed by the SEM monographs. The VSM studies confirm the magnetic behaviour by analyzing the change in magnetic saturation and coercivity of these nano materials.

Oxidation of Zn in UHV environment at low temperature

Thermal oxidation of polycrystalline Zn foils at 5×10−7Torr oxygen pressure and at room temperature, 50°C, 70°C, 90°C and 110°C was studied. In situ photoemission spectroscopy using synchrotron light with photon energy of 57eV was used to monitor the formation of ZnO and to determine the thickness of the oxide overlayer. At the initial oxidation, the oxidation rate follows a two-stage logarithmic equation and later trends to saturate at a certain thickness depending on the oxidation temperature. The saturated thickness was found to increase with the oxidation temperature. The two-stage oxidation process may be governed by two kinds of space charge presumably formed in the thin oxide overlayer.

Critical behaviour and magnetic properties of A–B spinel Zn xCu 1− xFe 2O 4

Polycrystalline Zn 0.6Cu 0.4Fe 2O 4 ferrites have been prepared using a solid-state reaction technique. Their structural and magnetic properties have been studied, using X-ray diffraction and Mössbauer and magnetic measurements. These results have been compared to a more general theoretical study, on Zn x Cu 1− x Fe 2O 4, based on mean field theory and high-temperature series expansions (HTSE), and extrapolated with the Padé approximant method. The nearest neighbour super-exchange interactions for the intra-site and the inter-site of Zn x Cu 1− x Fe 2O 4 spinel ferrites, in the range 0 ≤ x ≤ 1 , have been computed using the probability approach, based on Mössbauer data. The Curie temperature T C is calculated as a function of Zn concentration. The theoretical results obtained are in good agreement with the experimental results obtained by magnetic measurements.

Microstructure and nanohardness distribution in a polycrystalline Zn deformed by high strain rate impact

Polycrystalline Zn with an average grain size of about 300 μm was deformed by direct impact Hopkinson pressure bar at a velocity of 29 m/s. An inhomogeneous grain structure was found consisting of a center region having large average grain size of 20 μm surrounded by a fine-grained rim with an average grain size of 6 μm. Transmission electron microscopy investigations showed a significant dislocation density in the large-grained area while in the fine-grained rim the dislocation density was negligible. Most probably, the higher strain yielded recrystallization in the outer ring while in the center only recovery occurred. The hardening effect of dislocations overwhelms the smaller grain size strengthening in the center part resulting in higher nanohardness in this region than in the outer ring.

Structural, optical and magnetic properties of Eu-doped ZnO films

Polycrystalline Zn 1− x Eu x O ( x = 0, 0.02, 0.05) films were deposited on silicon (1 0 0) substrates by radio-frequency magnetron sputtering. The structural, optical and magnetic properties of the films were investigated. The results from both the X-ray diffraction and photoluminescence spectra reveal that Eu 3+ ions successfully substitute for Zn 2+ ions in the ZnO lattice. The magnetic field and temperature dependence of magnetization curves demonstrate that the Zn 0.95Eu 0.05O films are ferromagnetic at room temperature. No impurity phase was found in Eu-doped films with X-ray diffraction, Raman spectroscopy and zero-field-cooled measurements. The ferromagnetism is attributed to the intrinsic property of Eu-doped ZnO films and could be interpreted by the bound-magnetic-polaron model.